Flexible strain sensors have experienced growing demand due to their several potential applications, such as personalized health monitoring, human motion detection, structural health monitoring, smart garments, and robots. Recently, several academic results have been reported concerning flexible and stretchable strain sensors. These reports indicate that the materials and design methods have an important influence on the performance of strain sensors. Carbon-based nanomaterials including carbon-based nanofibers, carbon nanotubes, graphene, and carbon black nanoparticles play a key role in the fabrication of flexible strain sensors with excellent properties. In terms of design, carbon-based nanomaterials are generally combined with polymers to maintain the flexibility and stability of a strain sensor. Various combined methods were successfully developed using different assembly structures of carbon-based nanomaterials in polymers, such as uniform mixing and ordered structures, including films, fibers, nanofiber membranes, yarns, foams, and fabrics. The working mechanisms of the flexible strain sensors, including changing the conductive network between overlapped nanomaterials, tunneling effect, and crack propagation, are also different compared with that of traditional semiconductor and metal sensors. The effects of the carbon-based nanomaterial structures in polymers on the strain sensing performance have been comprehensively studied and analyzed. The potential applications of flexible strain sensors and current challenges have been summarized and evaluated. This review provides some suggestions for further development of flexible and stretchable strain sensors with outstanding performance.

The emergence of powerful nanomaterials characterization techniques promises to underpin a new range of advances in materials research. There have been significant developments in the characterization of the phase, structure, composition, and dynamics of materials at the nanoscale. Articles in this issue report recent advances in three areas: atom probe tomography, x-ray nanobeam scattering and diffraction, and x-ray photon correlation spectroscopy. Each of these provides three-dimensional insight into hard materials in ways that have been previously unavailable. Taken together, these emerging methods have the potential to provide new tests for materials theory and computation and to extend significantly the range of questions that can be answered in materials research.

Structural colors in nature have inspired the design of diverse photonic structures, which can interact with light via interference, diffraction or scattering. Among them, responsive soft material-involved photonic structures uniquely feature large volumetric changes upon external stimuli. The volumetric changes result in peak/valley shift of reflection spectra and perceptible color changes, providing responsive soft material-based structural color systems capability of serving as sensors for detecting chemical and biological analytes. Synthetic polymers and some natural materials are the most studied and utilized responsive soft materials for constructing structural color sensors, by tuning the thickness and morphology of formed films, or incorporating them into template structures, or their self-assembling. In this review article, structural colors in nature are firstly introduced, followed by discussing recent developments of promising responsive soft material-based structural color sensors, including the design of structural color sensors based on synthetic polymers and natural materials, as well as their applications for chemical sensing, biosensing, and multi-analyte sensing with sensor arrays. For specific sensing of chemicals and biomolecules, the sensing performance is evaluated in terms of detection range, sensitivity, response time, and selectivity. For multi-analyte sensing, cross-reactive structural sensor arrays based on simply a single soft material will be shown capable of discriminating various series of similar compounds. The future development of structural color sensors is also proposed and discussed.

The crystallization of amorphous complex oxides via solid phase epitaxy enables a wide range of opportunities in the formation of oxide materials in new geometries and with previously inaccessible compositions. Emerging methods for controlling crystallization from the amorphous form arise from recent advances in the deposition of amorphous oxides, the formation and placement of crystalline seeds, and have built on an expanded understanding of the kinetics of nucleation and crystal growth. Key discoveries include methods for the creation of epitaxial layers in perovskite, spinel, and pyrochlore complex oxides. The creation of nanoscale homoepitaxial and heteroepitaxial seeds has the potential to enable new directions in the integration of complex oxides with semiconductors and in devices based on oxygen ion transport. Future opportunities include the creation of complex oxides in morphologies and compositions exhibiting electronic, thermal, and magnetic phenomena enabling a variety of applications.

In this paper we discuss the various models that have been used to predict whether a material will tend to be ductile or brittle. The most widely used is the Pugh ratio, G/KG/K, but we also examine the Cauchy pressure as defined by Pettifor, a combined criterion proposed by Niu, the Rice and Thomson model, the Rice model, and the Zhou-Carlsson-Thomson model. We argue that no simple model that works on the basis of simple relations of bulk polycrystalline properties can represent the failure mode of different materials, particularly where geometric effects occur, such as small sample sizes. Instead the processes of flow and fracture must be considered in detail for each material structure, in particular the effects of crystal structure on these processes.